Progesterone is a naturally occurring steroid which is secreted from ovaries, placenta, and the adrenal gland and supports gestation. Progesterone is also available as a medication and is used for progesterone supplementation or replacement treatment for infertile women with progesterone deficiency. It is also indicated for the treatment of secondary amenorrhea. In the United States, pharmaceutical compositions of progesterone are available as vaginal gels, vaginal inserts, injections, or oral capsules. Progesterone is poorly soluble in water and bioavailability is low when taken orally.
The structure of progesterone is shown in FIG. 31.
Progesterone is known to exist in two polymorphic forms, I and II, with form II being a metastable form (J. Pharm Sci. 96:3419-31 (2007)). Cocrystals of progesterone have been disclosed in the literature. These include progesterone hydroquinone monohydrate (1:1:1) (J. Crys. Spec. Research, 19:983 (1989)); progesterone:indole (1:2) (Mar. 1, 2001 Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Crystallogr. (1981), 37, C56); pregnolone:progesterone (J. Pharm Sci. 96:3419-31 (2007)); resorcinol:progesterone (Acta Cryst. B31:637 (1975)) as are others (see PNAS 103:13216-13221 (2010) and US2005018041).
A cocrystal of a compound is a distinct chemical composition between the compound and coformer, and generally possesses distinct crystallographic and spectroscopic properties when compared to those of the compound and coformer individually. A coformer is also a compound and is often referred to as a “guest.” The compound which is not the coformer is often referred to as the “host.” Unlike salts, which possess a neutral net charge, but which are comprised of charge-balanced components, cocrystals are comprised of neutral species. Thus, unlike a salt, one cannot determine the stoichiometry of a cocrystal based on charge balance. Indeed, one can often obtain cocrystals having molar ratios of compound to coformer of greater than or less than 1:1. The molar ratio of the components is a generally unpredictable feature of a cocrystal.
Cocrystals have the potential alter physicochemical properties. More specifically, cocrystals have been reported to alter aqueous solubility and/or dissolution rates, increase stability with respect to relative humidity, and improve bioavailability of active pharmaceutical ingredients with respect to other cocrystals of such ingredients. The coformer, or guest, is often varied or selected for purposes of altering such properties.
The chemical composition of a cocrystal, including the molar relationship between the coformer and the compound (such as an API) can be determined by single crystal x-ray analysis. Where such an analysis is not available, often solution-state proton NMR is used to verify composition and identify molar ratio.
Cocrystal formation may be further confirmed by comparing solid-state analytical data of the starting materials with the corresponding analytical method collected of the cocrystal. Data from a co crystal will be represented by an analytical response that is not simply a linear superposition of the starting materials. For example, x-ray powder diffraction (XRPD) may be used for such comparison and the XRPD pattern of a cocrystal will differ from that of a physical mixture of the starting materials. Single crystal studies can confirm solid-state structure. In a cocrystal, the compound and the coformers each possess unique lattice positions within the unit cell of the crystal lattice. Additionally, indexing may be used to confirm the presence of a single phase.
A single crystal structure is not necessary to characterize a cocrystal. Other solid-state analytical techniques may be used to characterize cocrystals. Crystallographic and spectroscopic properties of cocrystals can be analyzed with)(RFD, Raman spectroscopy, infrared spectroscopy, and solid-state NMR spectroscopy, among other techniques. Cocrystals often also exhibit distinct thermal behavior compared with other forms of the corresponding compound. Thermal behavior may be analyzed by such techniques as capillary melting point, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) to name a few. These techniques can be used to identify and characterize the cocrystals.
For example, the entire XRPD pattern output from a diffractometer may be used to characterize a cocrystal. A smaller subset of such data, however, may also be suitable for characterizing a cocrystal. For example, a collection of one or more peaks from such a pattern may be used to characterize a cocrystal. Indeed, even a single XRPD peak may be used to characterize a cocrystal. Similarly, subsets of spectra of other techniques may be used alone or in combination with other analytical data to characterize cocrystals. In such examples of characterization as provided herein, in addition to the x-ray peak data, one also is able to provide the identity of the guest and host of the cocrystal and, often, their respective molar ratio as part of the characterization.
An XRPD pattern is an x-y graph with °2θ (diffraction angle) on the x-axis and intensity on the y-axis. These are the peaks which may be used to characterize a cocrystal. The peaks are usually represented and referred to by their position on the x-axis rather than the intensity of peaks on the y-axis because peak intensity can be particularly sensitive to sample orientation (see Pharmaceutical Analysis, Lee & Web, pp. 255-257 (2003)). Thus, intensity is not typically used by those skilled in the pharmaceutical arts to characterize cocrystals.
As with any data measurement, there is variability in x-ray powder diffraction data. In addition to the variability in peak intensity, there is also variability in the position of peaks on the x-axis. This variability can, however, typically be accounted for when reporting the positions of peaks for purposes of characterization. Such variability in the position of peaks along the x-axis derives from several sources. One comes from sample preparation. Samples of the same crystalline material, prepared under different conditions may yield slightly different diffractograms. Factors such as particle size, moisture content, solvent content, and orientation may all affect how a sample diffracts x-rays. Another source of variability comes from instrument parameters. Different x-ray instruments operate using different parameters and these may lead to slightly different diffraction patterns from the same crystalline cocrystal. Likewise, different software packages process x-ray data differently and this also leads to variability. These and other sources of variability are known to those of ordinary skill in the pharmaceutical arts.
Due to such sources of variability, it is common to recite x-ray diffraction peaks using the word “about” prior to the peak value in °2θ which presents the data to within 0.1 or 0.2 °2θ of the stated peak value depending on the circumstances. All x-ray powder diffraction peaks cited herein have are reported with a variability on the order of 0.2 °2θ and are intended to be reported with such a variability whenever disclosed herein whether the word “about” is present or not.
Raman spectroscopy and the related technique of infrared spectroscopy are additional techniques that may be used to characterize cocrystals together with or separately from x-ray powder diffraction. Raman and infrared peak intensity can be plotted versus “wavenumber.” A wavenumber has the units of inverse centimeters (cm−1). Wavenumbers are plotted on the x-axis intensity on the y-axis. As with x-ray powder diffraction plots, Raman and infrared peaks are recorded by reference to their x-axis (wavenumber) position rather than their intensity. Variation in the position of Raman and infrared peaks also exists and may be due to sample conditions as well as data collection and processing. The typical variability in such spectra reported herein is on the order plus or minus 2.0 cm−1. Thus, the use of the word “about” when referencing Raman and infrared peaks is meant to include this variability and all Raman peaks disclosed herein are intended to be reported with such variability whether the word “about” is present or not.
Thermal methods are another typical technique to characterize cocrystals. Different cocrystals of the same compound often melt at different temperatures. Variability also exists in thermal measurements, such as DSC, and may also be indicative of sample purity. Melting point, DSC, and hot stage microscopy, alone or in combination with techniques such as x-ray powder diffraction, Raman spectroscopy, infrared spectroscopy or some combination thereof, may be used to characterize cocrystals.
As with any analytical technique, melting points determinations are also subject to variability. Common sources of variability, in addition to instrumental variability, are due to colligative properties such as the presence of other cocrystals or other impurities within a sample whose melting point is being measured.